1. Field of the Invention
This invention relates in general to the field of communications channel receivers, and more particularly to an adaptively equalized receiver having a latching comparator circuit to continuously monitor the receiver output signal to improve equalization.
2. Description of Related Art
"Communication" is the exchange of thoughts, opinions, ideas, and information. It is the means to socialize, do business, educate, and entertain. Communication can take many forms, such as spoken words, written letters, or symbols. Although face to face communication is often desirable, it is often not possible due to geographical distance, time constraints, and an ever-increasing need for a high volume of information in today's society. It is for this reason that information, or data, is sent over communications "channels," via "signals."
A communications channel is a single path for transmitting an electrical signal, such as a twisted wire-pair cable, or a fiber optic line. A signal is a physical representation of data, such as the electrical pulses which are used to correspond to digital logic levels.
Signals are sent, or transmitted, in a tremendous variety of forms. For example, signals are used to send voice information over a telephone line; modems use signals to transmit data between computers; signals are constantly sent between the CPU and disk storage device in a personal computer; and signals representing images and sound are transmitted from a television camera on-site, to the television in a viewer's living room that could be thousands of miles away.
Signal distortion or degradation is a significant problem in the field of communications. Any real communications channel has transmission deficiencies, including various kinds of noise and interference, which distort the signal. For example, static noise (caused by natural electric disturbances in the atmosphere) and thermal noise (caused by the random motion of electrons in the channel) are present to some extent in any communications channel. Intersymbol interference (degradation caused by signals in different channels interfering with one another) can also be a major problem, especially given today's high volume of communications activity. In short, there are many reasons why a signal that is sent may be unrecognizable when it is received.
Thus, transmission deficiencies must be corrected so that the signal received is the same as the one that was sent, and valuable information is not lost. This correction can be accomplished by the signal receiver, through a process known as equalization.
Equalization is the process of correcting a channel for its transmission deficiencies, by introducing networks which compensate for attenuation and time delay problems in the signal. A properly equalized communications channel will significantly increase the likelihood of obtaining an accurate signal (i.e., the signal that was sent) at the receiving end of a communications network. An "equalizer" is a device used to accomplish equalization.
A filter can be used as an equalizer. Further, a filter may have a means of monitoring its own frequency response characteristics and a means of varying its own parameters by closed loop action, in order to attain optimal equalization. Such a self-adjusting filter is called an "adaptive filter," and it can be used in a channel receiver to attain "adaptive equalization." The parameters of an adaptive filter are typically adjusted by sampling the filter output at a predetermined rate, and sending this sampled output to some filter control means, which adjusts filter parameters accordingly via closed loop feedback.
However, such a scheme limits adjustment of the filter parameters to information available at the sample points. Any distortions in the signal that occur outside of a sample point will not be corrected until the time the next sample is taken, thus equalization of the channel will not be optimal. This could result in errors in the received signal.
In lower frequency systems such as modems, this problem is addressed by taking multiple samples within the baud period. This is called "fractionally spaced sampling." Fractionally spaced sampling reduces the possibility for signal errors because there is less time between sample points during which the signal is not monitored, and the receiver has more information (i.e., more samples) to use in reconstructing the signal that was sent. The multiple sampling ensures that when signal distortion occurs, the next sample point is never far away and thus the signal will be corrected quickly.
In higher data rate systems such as fast ethernet (e.g. the data rate is 100 Mb/s or more), however, fractionally spaced sampling becomes unrealistic due to high costs and physical limitations of receivers. Thus, higher data rate systems typically sample only once in the baud period; this is called "baud rate sampling."
Furthermore, in baud rate sampling systems, the receiver is usually designed so that sampling occurs at the center of the baud period, in order to minimize the likelihood of incorrectly reading the level of the signal. Nevertheless, baud rate sampling can be more susceptible to errors in reconstruction of the signal since only a single sample is used to determine the value of that signal each baud period.
Thus, it can be seen that there is a need for a receiver that improves equalization of received signals.
It can also be seen that there is a need for a receiver that corrects for channel transmission deficiencies.